This invention relates to an accessory which can be used with a spectrometer in order to carry out attenuated total reflectance (ATR) measurements. In particular the invention relates to an accessory which can carry out horizontal attenuated total reflectance (HATR) measurements using a crystal in which multiple reflections of the analysing radiation occur.
In the field of spectroscopy such as FT-IR spectroscopy, it is known when poorly reflecting samples are being analysed to use what is called an attenuated total reflectance (ATR) technique. This usually involves mounting in the spectrometer an accessory which incorporates a crystal which utilises total internal reflectance or attenuated total internal reflectance. The sample is located in contact with a crystal which is to be analysed and contact between the sample and the crystal is usually maintained by the application of pressure to the crystal or the sample in the case of a solid sample.
One known type of crystal is an elongate crystal of generally trapezoidal shape which is used to carry out what is commonly known as horizontal attenuated total reflectance measurements. When using such a crystal analysing radiation enters the crystal through one of its inclined end faces and undergoes multiple internal reflections along the length of the crystal before exiting the crystal through its other inclined face. Such a crystal can be used to carry out an analysis of both solids and liquids. The sample under investigation is placed in close contact with one of the plane surfaces of the crystal and the analysing radiation slightly penetrates the sample at its point of contact with the crystal as the radiation undergoes multiple reflections along the length of the crystal. By analysing the radiation exiting from the crystal it is possible to produce a spectrum of the sample under investigation.
It is known to vary the angle of incidence of the radiation on the crystal end face in order to change the penetration depth of the analyzing radiation and it is known also to use crystals of different dimensions according to the particular analysis being carried out. When a change of crystal or a change of angle of incidence is required it is usually necessary to change the optics of the arrangement in order to ensure the analyzing radiation enters the crystal at the correct angle. Conventionally this has been carried out manually and is a somewhat tedious operation. Furthermore the adjustment are not accurately repeatable and this can affect the accuracy of measurements.
Additionally, it has been difficult to achieve a reliable and consistent way of applying force to the crystal or sample in order to ensure the appropriate contact between the crystal and the sample under investigation.
The present invention is concerned with an accessory for carrying out horizontal attenuated total reflection measurements in which optical elements can be adjusted automatically when a change of crystal is required.
Additionally, the invention provides an arrangement in which a reliably reproducible force can be applied to the crystal or sample to maintain contact between the crystal and the sample under investigation.
According to the present invention there is provided an accessory for use with a spectrometer which has a sample station in which the accessory can be located, said accessory comprising a mounting for receiving an ATR crystal, first optical elements for directing an incoming beam of analysing radiation to said crystal, second optical elements for directing a beam of radiation exiting the crystal to an outlet, at least one of the first optical elements and at least one of the second optical elements being pivotable, and means operable to cause equal and opposite pivoting movement of said pivotable elements so that the beam of analysing radiation is caused to be incident on said crystal at an angle appropriate for particular crystal employed or the measurement to be made.
The first optical elements may include a first mirror fixed to a first plate which is pivotally mounted on the housing of the accessory, the second optical element may include a second pivotable mirror mounted on a second plate which is pivotally mounted on the housing of the accessory. The pivoting means may comprise a rotatable cam member disposed at least partially between and in contact with said plates. The cam member may include a plurality of angularly spaced portions of different radii whereby rotation of the cam member causes said plates to assume an orientation which is different according to which portion of the cam member contacting said plates.
The cam member may be rotatable by a motor. The motor may be a stepper motor.
The first mirror may be a toroidal mirror and the second mirror may be a planar mirror.
The accessory may include electrical connectors which when the accessory is located in the sample station make contact with a connector on the spectrometer and the crystal it may have incorporated therein a data storage medium in which is stored data indicative of the crystal, the arrangement being such that when the accessory is located in the sample station the processor of the spectrometer can read said stored data and can transmit energising signals to said motor to cause the pivotable mirrors to be adjusted to a position appropriate to the crystal.
The first optical elements may also include a fixed plane mirror and a fixed toroid/ellipsoidal mirror, and the second optical elements may also include a fixed toroidal/ellipsoidal mirror and a fixed planar mirror.
The accessory may include an arm mounted on said housing, said arm including pressing means contactable with said crystal or a sample located adjacent said crystal in order to apply pressure to said crystal to maintain contact between the crystal and a sample under investigation. Pressure measuring means may be associated with said arm. The pressure measuring means may comprise a strain gauge. The strain gauge may be electrically connectable to said processor of said spectrometer in order to enable data relating to the applied pressure to be transmitted to said processor. A display means may be provided to display the sensed pressure thereby enabling an operator to accurately set the optical pressure.